The present invention relates to thin film electroluminescent (TFEL) panels which include a layer of electroluminescent material sandwiched between dielectric plates containing orthogonally disposed electrodes to form a matrix of pixels.
TFEL panels may conventionally comprise a matrix of pixels formed by the intersections of a plurality of row electrodes and a plurality of column electrodes. These electrodes are situated on plates disposed on either side of a thin electroluminescent layer of material such as zinc sulfide. The row electrodes are energized in turn, usually from the top of the screen to the bottom, once per frame with a voltage often termed a "write" voltage. Simultaneously with the energization of each row electrode with the write voltage, selected column electrodes are energized with a modulation voltage which raises the potential across the electroluminescent film to a level above its threshold of luminescence, thus illuminating selected pixels in that row. Once all the rows have been energized in this fashion, a frame of data is completed. At the end of a frame of data it is necessary to remove the accumulated charge across the screen. This is accomplished by applying a "refresh" voltage pulse to all of the row electrodes simultaneously at the end of a frame of data. The refresh pulse is opposite in polarity to that of the write voltage pulse and is approximately equal in amplitude to the combination of the write voltage pulse plus the modulation voltage pulse. A typical driving architecture for such a system is described in copending patent application No. 729,974 which is assigned to the same assignee.
Over an extended period of time a phenomenon common to TFEL screens takes place in which certain parts of the screen age faster than others. This aging phenomenon is due primarily to a concentration of charge at the internal interfaces of the electroluminescent film and the dielectric layers. The charge accumulation is caused by the time asymmetry between the write and the refresh pulses. It has been observed that the differential aging effect is most pronounced at the top and at the bottom of the screen where the time asymmetry is at a maximum. In the center of the screen the differential aging effect is at a minimum because the timing of the write and refresh pulses is very nearly symmetrical. That is, the row electrodes in the center of the screen are alternately charged with the write voltage about halfway through the frame, and are discharged with the refresh voltage at the end of the frame. Thus, for half of the frame these rows experience the residual effects of a voltage of one polarity and for the other half of the frame (which may be the first half of the next frame) they experience the same effect from a voltage of the opposite polarity. The result of the differential aging effect is that any pattern on the screen which has been displayed continuously for an extended period of time is likely to remain as a faint image on a dark screen and thus constitute a source of visual background distortion.